FIG. 1 shows an organic thin-film transistor of prior art, having a so-called “high-gate” and “low-contact” structure. In this instance, said transistor 1 comprises:                a lower substrate 2, having two electrodes 3, 4, a first source electrode 3 and a second drain electrode 4, formed thereon,        a semiconductor layer 5 made of a semiconductor material and deposited on lower substrate 2, source electrode 3 and drain electrodes 4,        and a dielectric layer 6 made of a dielectric material and deposited on semiconductor layer 5, and having a gate electrode 7 formed thereon.        
The transistor effect is obtained, as known per se, by applying a voltage between gate electrode 7 and lower substrate 2 to create, in semiconductor layer 5, a conduction channel 8 between source electrode 3 and drain electrode 4.
However, field-effect transistors integrating a semiconductor layer made of an organic material comprise a dielectric layer having imperfect insulating properties so that a leakage current 9, schematically shown in FIG. 1, appears between gate electrode 7 and electrodes 3 and 4, thus altering the operation of said transistor.
Indeed, leakage current 9, which is a current flowing through dielectric layer 6 from gate electrode 7 to source 3, strongly degrades current Ioff of the field-effect transistor.
When the leakage current is too strong, the transistor breaks down and current Ion and Ioff is no longer measured, the only measured current being the leakage current, since the field effect of the transistor has then disappeared.
Such a degradation of current Ioff of the field-effect transistor, due to the leakage current appearing between the gate electrode and the source electrode in the dielectric layer, forbids the use this type of organic field-effect transistor to form electric circuits.
To decrease the leakage current, it may be envisaged to modify the intrinsic properties of dielectric layer 6 and/or to increase the thickness of said dielectric layer 6. “Intrinsic properties of dielectric layer 6” means the insulating properties of said dielectric layer and/or the quality of the deposited dielectric layer.
However, modifying the intrinsic properties of dielectric layer 6 or increasing its thickness increases the manufacturing costs of such transistors and also modifies the other transistor characteristics, such as, particularly, the capacitance of the dielectric layer, which is then capable of generating stray capacitances also altering the proper operation of the transistor.